1. Field of the Invention
The present invention relates to a motor drive control apparatus having a feedforward control element, and more specifically, to a motor drive control apparatus for improving the follow-up response of a motor when a speed increase/decrease command is issued.
2. Description of the Related Art
FIG. 1 is a block diagram showing the arrangement of an example of a prior art motor drive control apparatus and the operation thereof will be described below. Present position data Pa of a motor 11 is obtained when a signal from a position detector 10 mechanically coupled to the motor 11 is processed by a position detecting section 5. A positional error amount Pe is obtained when the present position data Pa of the motor 11 is subtracted from the value of a movement position command Pr which is generated by a position command generating section (not shown in FIG. 1). A speed command Vr is generated in a speed command calculation section 1 based on the amount of the positional error Pe. Further, present speed data Va of the motor 11 is determined in a speed detecting section 3 based on an expression (1) from the present position data of two present positions continued in time. EQU Vai=(Pai.sub.+1 -Pai)/.DELTA.Tps (1)
where,
i: integer showing a time series number PA1 .DELTA.Tps: present position data detecting cycle PA1 speed control cycle (torque command generating cycle) is twice as long as that of a current control cycle (PWM voltage command generating cycle); PA1 calculation dead time for each control loop is assumed to be zero; and PA1 torque command generated in a speed control loop is not commanded to a current control loop until a next speed control cycle begins. In FIG. 2A, a torque command generated in the speed control cycle starting from a time t0 is assumed to be made effective from the first current control cycle included in the speed control cycle time starting from a time t2.
A speed error amount Ve is obtained by taking the difference between the present speed data Va and the speed command Vr which are obtained as described above. A torque command Tr is generated by carrying out a known proportional plus integral control calculation with respect to the speed error amount Ve in a speed PI-calculation section 2.
Assuming that the motor 11 is an AC brushless motor (synchronous motor), the relationship between a current flowing through the motor 11 and a generated torque will be described. Assuming that the permanent magnet mounted to a rotor has a maximum densits Be, flux densities Bu, Bv and Bw perpendicular to the respective phases U, V and W of the motor 11 are represented by expressions (2) to (4), respectively. EQU Bu=Bo.multidot.cos .theta. (2) EQU Bv=Bo.multidot.cos (.theta.+2.pi./3) (3) EQU Bw=Bo.multidot.cos (.theta.+4.pi./3) (4)
Further, currents Iu, Iv and Iw having a maximum value Io flow to the windings of the three phases of a stator, as shown in expressions (5) to (7). EQU Iu=Io.multidot.cos .theta. (5) EQU Iv=Io.multidot.cos (.theta.+2.pi./3) (6) EQU Iw=Io.multidot.cos (.theta.+4.pi./3) (7)
Assuming that the winding of the stator has an average radius r and an effective length le, a torque produced in the motor 11 is represented by an expression (8) in accordance with Fleming's left hand rule. ##EQU1## Therefore, the maximum value Io of a current flow must be as shown in an expression (9) in order to produce a rotational torque in coincidence with the torque command Tr in the motor 11. EQU Io=Tr/Kt (9)
A phase current command calculation section 4 for each phase first determines the position .theta. of the U-phase winding of the stator at a particular time from the present position data Pa of the motor 11. Next, the command values Iur and Ivr of currents which to the U- and V-phase windings of the motor are derived from expressions (10) and (11) based on the aforesaid point of view. EQU Iur=Tr.multidot.sin .theta. (10) EQU Ivr=Tr.multidot.sin (.theta.+2.pi./3) (11)
On the other hand, the data of a current sensor for sensing the currents of the U- and V-phase windings of the motor 11 is transferred to a current detecting section 6, which generates feedback current data Iua and Iva. Current error amounts Iue and Ive of the respective phases are determined by obtaining the differences between the current command values Iur and Ivr and the feedback current data Iua and Iva. Voltage commands Vur and Vvr to be imposed on the windings of the respective phases are generated by causing the current error amounts Iue and Ive of the respective phases to be subjected to a known proportional plus integral control calculation in a PI-calculation section 7. Since a vector sum of the currents flowing through the respective phases is controlled to be zero in the motor 11, when the current command values for two phases of the three phases are determined, a command value for the remaining one phase can be simply determined. Then, a PWM control section 8 is input with the voltage commands Vur and Vvr to be imposed on the U- and V-phases determined by the proportional plus integral calculation carried out in the PI-calculation section 7 and the voltage commands Vur and Vvr and to be imposed on a W-phase. The PWM control section 8 carries out a known PWM control operation based on these imposing voltage commands Vur, Vvr, Vwr so that a voltage output from an electrical power amplifier 9 is controlled.
FIGS. 2A to 2C are diagrams showing an example of the relationship between the torque command, the current command, the PWM voltage command for a single phase, and the feedback current and the current error amount. These figures are described based on the following premise:
Under the aforesaid premise, as shown in FIG. 2A, it is assumed that a torque command Tr0 changing stepwise from zero to a certain value in the speed control cycle starting from the time t0 and current commands Iur0 and Iur1 corresponding to it have been generated. The current command Iur0 is made effective in the first current control cycle at and after the time t2. A difference between the stepwise input current command Iur0 and an actually detected feedback current value, i.e., a current error amount Iue0 is calculated in this current control cycle and subjected to the known proportional plus integral control calculation to thereby generate a PI-calculation voltage command Vur0. For the convenience of the descriptions, since it is assumed that this processing for the current control calculation is started from the time t2 and the calculation dead time is zero, FIGS. 2B and 2C are made on the basis that the stepwise input current command Iur0, current error amount Iue0 and PI-calculation voltage command Vur0 are fixed at the time t2. A voltage output from the electrical power amplifier 9 is controlled by the PWM control section 8 which carries out the known PWM control operation based on the PI-calculation voltage command Vur0, and as a result a current flows through the motor 11 and a feedback current shown by a thick line in FIG. 2B is detected. Thereafter, similar commands are calculated and generated, respectively, with respect to torque commands generated at the times t2, t4, . . . and a control is carried out such that a finally commanded torque is produced to the motor 11.
As described above, a current, a speed and a position are feedback-controlled by a current feedback loop, a speed feedback control and a position feedback control respectively so that amounts of the current error, the speed error and the positional error of the respective phases become zero.
A follow-up error of a motor to a command is made large particularly when a torque is abruptly changed because a gain in a control loop cannot be set infinitely large and there is a control dead time element. Since, however, a prior art motor drive control apparatus is arranged as described above, there is no adjustment means except the adjustment of each loop gain to adjust a speed response and a current response by which the command follow-up property of the motor is determined. In this case, since a gain is adjusted and set in consideration of a steadily control operation, there is no means for improving the follow-up property only when a torque is abruptly changed. Thus, when a control gain is set to a large value to improve the follow-up property at the time the torque is changed, problems such as the occurrence of hunting and vibration, and the like are caused because the set gain is too large as a gain at a steady time.